The present invention relates to improvements in electronic devices implantable in living bodies and more particularly to an improved hermetically sealed package for housing such electronic devices.
Stimulators which are to be implanted in living bodies and powered from external information sources must be housed in packages of biocompatible material. Such packages must protect the electronic circuitry within the implanted stimulator from body fluids and ions so that the circuitry can survive for extended periods without any significant changes in performance.
Early efforts at packaging implantable electronic devices, beginning with miniature radio telemetry units in 1957 and cardiac pacemakers in 1958, employed polymer packages. For the next 15 years, the field of implant packaging was dominated by polymers, principally epoxies, elastomers, and Teflon. Unfortunately, implant failures were common with such packages. Finally, it became clear that packages formed even in part of polymers when implanted in living bodies did not provide a permanent barrier to water which passed through the polymer compounds as water vapor and condensed in cavities or on the surfaces of electronic components housed in such packages.
Besides water leakage, polymer packaging presents other disadvantages. Hard polymer materials, such as epoxies, tend to change volume during curing, often shrinking and exerting damaging stresses on components of a polymer package. Also, water, lipids and other components of body fluids may be absorbed by such polymers causing delayed volume changes and package damage. In addition, sometimes polymers evolve gases or acids during curing while some non-adherent coatings, like Teflon, form channels that can quickly wick fluids by capillary action to the most vulnerable circuit components of the implanted stimulator. Such problems exist even when a polymer is only part of a packaging system that includes a hermetic capsule for electronic components. Also, it has been found that polymer coatings sometimes limited or prevented effective hermeticity leak testing of a metal, glass or ceramic portion of a packaging system because of out-gassing from the polymer. In view of such shortcomings, polymer encapsulation is no longer considered an acceptable packaging technique for chronic implants. For example, all pacemakers are now hermetically sealed in metal packages.
In that regard, in the early 70's, the first commercially successful hermetically sealed in metal pacemaker was introduced. Today, the most commonly used metals for implantable packages are titanium, stainless steel and cobalt-chromium alloys. These metals are biocompatible and corrosion resistant. Normally, the package consists of two parts welded together to insure hermeticity. The electrical components inside the package are connected to stimulating leads by hermetic feedthroughs. However, where there is a need to couple an alternating electromagnetic field to an internal pickup coil, the metal becomes a hinderance. Transmission of power is substantially reduced by eddy currents generated in the metal due to the alternating electromagnetic field. To solve that problem, receiving coils are often placed outside the metal package, increasing the size of the implant.
It is known that glasses and ceramics are transparent to alternating electromagnetic fields and that receiving antennas can be placed inside a hermetic zone of a ceramic or glass package, creating an overall smaller implant device and reducing the possibility of antenna failure due to saline leakage. Glasses and ceramics are inert and highly insoluble, which are favorable characteristics for long term implant materials. Unfortunately, however, because glasses and ceramics are inelastic, they are subject to fracture not only from mechanical shock but also from differential thermal expansion if even a moderate temperature gradient exists thereacross. Therefore, welding is not a practical method of sealing glass or ceramic materials. Instead, virtually the entire package and its contents must be raised to the melting temperature of the glass or ceramic or metal braze used to effect a sealing of the glass or ceramic package. Such sealing methods are unsatisfactory. All known biocompatible glasses and ceramics are characterized by high sealing temperatures that will damage electronic components commonly included in electronic devices implanted in living bodies. Low melting temperature glasses all have the property of being corroded by body fluids. Further, metal or glass frits and solders useful in brazing glasses and ceramics and having melting temperatures below the thermal damage limits of implanted electronic components are either not biocompatible or corrode easily in body solutions. Therefore, packages composed entirely of ceramic and/or glass are not considered practical for such implant applications. Also, in prior ceramic and glass packages, the metal solder sealing the main body and cap portions of such packages has formed a closed loop very close to, coaxial with or in a plane parallel to the receiving coil comprising the antenna for the electronics housed in the implantable package. Thus configured, the closed metal loop of solder has acted as a shunt to the alternating electromagnetic fields impressed upon the package to transmit power and/or data to the implanted electronics. This has resulted in the generation of undesired heat within the package and the reduction of power transfer efficiency.
Therefore, there is a continuing need for a non-conducting hermetically-sealed package for electronic components which are damageable by high temperatures. The present invention satisfies that need.